The impact of clay dispersion and migration on soil hydraulic conductivity and pore networks

Highlights

• The reduction of K_sat was related to the decreased EC and increased sodicity.

• The dispersed clay in leachate was not necessarily corresponding to K_sat reduction.

• Low density soil columns maintained higher K_sat compared to the compacted soils.

• High porosity and macropores enabled dispersed clay to move through soil column.

Abstract

Using saline and sodic waters for irrigating agricultural lands has increased worldwide, which may cause adverse effects on soil porosity, structural stability and permeability. However, few studies have investigated the effect of clay swelling, dispersion and migration on the reduction of saturated hydraulic conductivity (K_sat) by analysing the total porosity and pore size change through the profile. In this study, two soils with contrasting mineralogy was used and packed at two different bulk densities (1.0 and 1.2 g cm⁻³). Soil columns were leached with three types of sodium adsorption ratio (SAR) solutions, i.e. good quality water (GQW), SAR 10 and SAR 50 solutions, before distilled water (low ionic strength) was leached through. The results indicated that the application of distilled water decreased K_sat for all soils, particularly in the initially sodic water treated columns. The K_sat of leaching with distilled water in low density columns were generally higher, but showed more degree of reduction than more compacted columns, mainly related to the high initial porosity and macropores that enabled dispersed clay to move through the soil column and out in the leachate. The application of distilled water significantly reduced the proportion of macro and mesopores, whereas increased the proportion of micropores in both soils, particularly for soil columns initially treated high SAR solutions. When the dispersed clay was trapped within the column, the depth of clay accumulation was affected by a combined effect of packing density, soil type and water quality.

Introduction

The saturated hydraulic conductivity (K_sat) of soils is a key hydraulic property for soil management, as it affects global aspects from agriculture to forestry, to slope stability and flood protection (Liu et al., 2020, Vergani and Graf, 2016). The K_sat of soils largely depends on soil porosity and pore size distribution, whereas in irrigated soils, these two parameters are influenced by the quality of irrigation water applied (Alaoui et al., 2011, Speirs et al., 2011). Worldwide, marginal quality water is used as an important agricultural water resource and its amount continues to increase (Dang et al., 2018b). Irrigation with saline-sodic water can result in the accumulation of Na⁺ in the soil profile, where soil becomes sodic, and accompanied with the effects of low electrolyte concentration in the pore water, leads to low infiltration, hardsetting, sealing and poor aeration of the rooting zone and impairs root growth and plant establishment (Levy, 2012, Rengasamy and Olsson, 1991, Van Hoorn and Van Alphen, 1994).

The potential for structural degradation is more severe when irrigation solutions have high values of sodium adsorption ratio (SAR) or cation ratio of soil structural stability (CROSS), but occurs when the electrolyte concentration is less than a critical value, i.e. threshold electrolyte concentration (C_TH) (Bennett et al., 2019a, Marchuk and Marchuk, 2018, Quirk and Schofield, 1955, Sumner, 1993). As a consequence, soil structure degrades mainly through the following processes (Aringhieri and Giachetti, 2001, Bennett et al., 2019a, Mace and Amrhein, 2001, Minhas et al., 1998, Shainberg and Letey, 1984, Sumner, 1993): (i) dispersion and subsequent migration of dispersed clay that block soil pores, or (ii) intra-crystalline (depending on 2:1 smectite clay presence) and/or inter-crystalline swelling due to the development of diffuse double layer, which narrows soil pores. Marchuk and Marchuk (2018) found the K_sat of a swelling soil decreased more than non-swelling soil when the CROSS value of treatment solutions increased from 6 to 15, while no visible suspended clay was observed in the swelling soil column effluent. They found the percentage of macropores decreased dramatically with the increase of CROSS, while the micropore increased. Additionally, reduction in K_sat of up to 90% without dispersion was observed for soils with vertic properties, attributed to swelling and disaggregation process as a function of water quality (Dang et al., 2018a). Minhas and Sharma (1986) examined the effects of applying saline-sodic water with different SAR (5–45) and electrolyte concentrations (15–90 mmol_c L⁻¹) followed by the application of distilled water to two soils (a sandy loam and a clay loam). The authors reported a significant reduction in hydraulic conductivity, which was greater in the clay loam soil due to the relatively higher clay content, and with increasing SAR and decreasing electrolyte concentration. The study attributed the reduction in K_sat to dispersion of clay particles and subsequent pore clogging. A later study from Mace and Amrhein (2001) examined lower SAR values (1–8) and a wider range of electrolyte concentrations (from 0 to 100 mmol_c L⁻¹) on hydraulic conductivity, clay dispersion and pore blockage. The authors found that clay dispersion, pore blockage and consequently K_sat under sodic conditions is also significantly affected by clay mineralogy. These studies all consider K_sat as an indicator of clay dispersion and pore blockage. However, few studies have investigated the interactions between soil porosity and the movement of the dispersed clay and the potential zones of pore blockage within the soil profile. This has important implication for the management of irrigated soils as the clay migration and zones of blockage will impact on the potential for surface sealing, crusting and hardsetting behaviour.

Hence, the objective of the present study is to evaluate the relationships between porosity, clay dispersion and migration, and change of K_sat when the saline-sodic soil leached with distilled water.